Thrombotic complications constitute major life-threatening conditions for both the aging population and young adults (Hansson 2006; Libby 2005). One underlying cause is the activation of the blood coagulation cascade and fibrin deposition, which can generate occlusive blood clots and impede blood flow, leading to thromboembolism, deep-vein thrombosis, ischemic heart diseases or stroke (Libby 2005). Elevated levels of thrombin resulting from an activated coagulation cascade are associated with almost all inflammatory conditions ranging from arthritis (Morris 1994; Busso 2002; Kitamoto 2008; Flick 2011), pulmonary fibrosis (Ludwicka-Bradley 2004; Vergnolle 2009; Bogatkevich 2011), inflammatory bowl diseases (Vergnolle 2009; Saibeni 2010) to cancer (Khorana 2004; Karimi 2010). Active deposition of fibrin occurs within atherosclerotic plaques, which promote the progression of atherosclerosis toward occlusive eruptions (Duguid 1946; Peters 2009). Extravascular fibrin deposition is a major pathogenic factor for chronic synovial inflammation in arthritis, especially in osteoarthritis and rheumatoid arthritis (So 2003; Busso 2002). Thrombosis increases the lethality of many human cancers (Agorogiannis 2002; Khorana 2004; Rak 2006; Lorenzet 2002; Ornstein 2002; Nierodzik 2005; Karimi 2010) and infectious diseases (Levi 2003; Marsden 2003; Opal 2003). Such widespread occurrence and unmet medical needs have propelled a continued search for more efficacious, safe and cost-effective anti-coagulant and anti-thrombotic therapies (Gross 2008; Hoppensteadt 2008; Theroux 2000; Warkentin 2004) and a better understanding of blood coagulation biochemistry (Mann 2006; Kamath 2008; Bock 2007; Wood 2011). These latest research advances present a unique opportunity for the design, discovery and development of anti-thrombotic agents specific to the localized characteristics of vascular lesions, atherosclerotic plaques and inflamed joints and tissues.
The blood coagulation cascade is triggered by the expression of tissue factor on injured vasculatures or tissue cells (Mann 2006; Mann 1988), e.g. at sites of atherosclerotic lesions (Libby 2005) and within inflamed joints (Busso 2002) or invasive tumors (Khorana 2004; Karimi 2010). All coagulation pathways converge on the prothrombinase assembly, which rapidly converts prothrombin into the ultimate protease thrombin responsible for the formation of the blood (fibrin) clot (Mann 1987; Mann 1988). Generation of thrombin requires finely orchestrated cleavages of two peptide bonds in prothrombin by the prothrombinase composed of the serine protease factor (F) Xa, and the protein cofactor Va, which are assembled on appropriate membranes in the presence of Ca2+ ions (Mann 1988; Mann 1987; Wood 2011). Depending on the physiological contexts, prothrombin activation can also accumulate thrombin in anti-coagulant and anti-inflammatory forms (Nesheim 2003; Hackeng 1996; Asai 2004; Nishimura 2007), especially in complexes with membrane-bound thrombomodulin (Nesheim 2003) instead of the fully-procoagulant and circulating form needed for the rapid formation of platelet-rich haemostatic plugs (Wood 2011).
The current generation of coagulation inhibitors, among which many are direct thrombin or FXa inhibitors, are administered and active systemically (Vorchheimer 2002; Hoppensteadt 2008; Gross 2008; Gresele 2002), and as such can cause either bleeding side effects or rebound coagulation and re-occlusion after cessation of therapy (Gresele 2002; Fareed 2008; Weitz 2002; Vorchheimer 2002). By design, these coagulation inhibitors reduce and deplete the levels of thrombin non-discriminatively, irrespective of the pro-coagulant or anti-coagulant activities of thrombin (Nesheim 2003). These complications point to the need for more effective and selective anticoagulants, especially for locally-active thrombin inhibitors to prevent pathogenic blood coagulation only at sites of occlusive vascular and/or tissue injury (Riewald 2002; Khrenov 2002; Libby 2002; Busso 2002).